The development of high energy battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials, such as lithium, sodium, and the like, and the efficient use of high energy density cathode materials, such as manganese dioxide, FeS.sub.2, CuO, CuS, Co.sub.3 O.sub.4, PbO.sub.2, oxyhalides, and poly-carbon fluorides having the general formula (C.sub.y F.sub.x)n wherein y is 1 or 2, x is greater than 0 up to about 1.5 and wherein n refers to the number of monomer units which can vary widely. It has, therefore, been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, to turn to the use of nonaqueous electrolyte systems. In the manufacturing and assembly of these high energy cell systems, the anodes, such as lithium, are usually soft materials that are stamped from strips or sheets into circular discs for use in cylindrical nonaqueous cells.
In the manufacture of soft strips of lithium, it has been proposed in the prior art, specifically U.S. Pat. No. 3,721,113, that thin continuous lithium strips can be produced by cold rolling lithium metal while it is compressed between smooth surfaces of a solid polymeric composition, which composition is nonreactive with lithium and has a critical surface tension of not over 46 dynes per centimeter at 20.degree. C. The use of the polymeric sheet material is essential so as to prevent the sticking of the lithium to the metal surfaces of the roller. Once continuous lithium strips are produced another problem encountered is in the cutting of the lithium metal into a plurality of pieces which can be employed as lithium anodes in various types of cell systems. To overcome this problem, it is disclosed in the prior art, specifically U.S. Pat. No. 4,060,017, that a flexible film, preferably of plastic, be interposed between the blade of a cutting device and a lithium strip such that when the blade is forced against the anvil with sufficient force to cut the lithium, the film prevents contact of the blade with the lithium. This will prevent any buildup of lithium being developed on the blade which would occur generally after only a few cutting operations.
Due to the relative softness of the lithium material, the lithium anode circular discs are generally stamped from a strip having a width larger than the diameter of the desired lithium disc to ensure that the discs can be produced on a continuous and reproducible basis without distortion. This results in a scrap loss of lithium that can be as high as 62 percent.
Japanese published unexamined application JA 82/128459 discloses the fabrication of lithium anodes in the shape of squares which can be cut from a strip of lithium having a width equal to the length of a side of the lithium anode. Although this method will reduce the scrap loss of lithium when producing square lithium anodes from lithium strips, it has the disadvantage in that for a fixed thickness, too much of the lithium is removed when such anode is employed in a cylindrical cupped container. For example, a square lithium anode would have to be increased in thickness by 57 percent of the thickness of a circular anode to equal the amount of lithium contained in the circular anode having a circumference circumscribed about the square anode. Miniature electronic devices which require the use of thin but powerful cylindrical electrochemical cells may not be able to tolerate this increase in anode height in the cell.
One of the objects of the present invention is to provide a polygonal shaped anode disc, such as lithium for use in cylindrical (button, coin, etc.) cell containers, that can be easily fabricated from lithium strips with minimum scrap loss.
Another object of the present invention is to provide a polygonal shaped anode disc having a circumference composed of at least four segments of which at least two of the segments are straight segments.
Another object of the present invention is to provide a polygonal shaped lithium anode disc having a surface area of at least 82 percent of the area of a circle circumscribed about said polygonal shaped anode.
Another object of the present invention is to provide a method for producing polygonal shaped anode discs, such as lithium anodes for use in a cylindrical cupped container, on a continuous basis from a strip of the anode material with minimum scrap loss.
The foregoing and additional objects will become more fully apparent from the following description and drawing